NEW 40AR/39AR AGE DETERMINATIONS FROM THE K/T BOUNDARY INTERVAL IN THE RATON BASIN, NORTHERN NEW MEXICO—CONTINUING STUDIES OF SEDIMENTATION RATES OF THE K/T SEQUENCE

In an abstract for the "Snowbird IV" meeting in Vienna in 2001, we reported a new age determination from sanidine recovered from a kaolinitic tonstein parting that lies only 25 cm beneath the K/T boundary claystone near Raton, NM. Tonsteins are considered to result from the alteration of volcanic ash layers deposited in coal swamps. We have since made further age determinations on the tonstein from this site, known as Canadian North (CN), and from another locality nearby, called Canadian South (CS), using a monitor age of 27.84 Ma for the Fish Canyon Tuff (FCT). We simultaneously dated tektite glass from Haiti using the same standards. Our previously reported 40Ar/39Ar age for the CS tonstein was 65.45±0.11 Ma; an age of 65.15±0.23 Ma was obtained for the Haitian Tektite glass.

We have calculated new ages for the K/T boundary interval in the Raton basin using high-purity sanidine separates obtained from the tonstein beneath the K/T boundary at these two localities, again using the monitor age of 27.84 Ma for the FCT. At CS, the tonstein is 90 cm below the boundary and at the CN site it is 25 cm below the boundary. Three sanidine separates were analyzed from the CN site that yielded ages of 65.47±0.11 Ma, 65.20±0.17 Ma, and 65.07±0.20 Ma, and four sanidine separates from the CS site yielded ages of 65.49±0.10 Ma, 64.88±0.30 Ma, 65.36±0.21 Ma, and 65.40±0.10 Ma. The Haitian tektite glass was also reanalyzed and yielded. an age of 65.06±0.21 Ma. Currently, we intend to redate this tonstein and the tektite glass, but using a larger quantity of sanidine to reduce the error in the analyses. These new data will be used to determine rates of coal formation and deposition of carbonaceous shale and mudstone in the K/T sequence from the Raton basin.